US 20050093092 A1
A resistive cross point array memory device comprising a plurality of word lines extending in a row direction, a plurality of bit lines extending in a column direction such that a plurality of cross points is formed at intersections between the word and bit lines, and at least one memory element formed in at least one of the cross points. The memory element comprises a first tunnel junction having a bottom conductor, a top conductor, a barrier layer adjacent the bottom conductor, and wherein the bottom conductor comprises a non-uniform upper surface.
1. A resistive cross point array memory device, comprising:
a plurality of word lines extending in a first direction;
a plurality of bit lines extending in a second direction such that a plurality of cross points is formed at intersections between the word and bit lines;
at least one memory element formed in at least one of the cross points, the memory element comprising a first tunnel junction, the first tunnel junction comprising a bottom conductor, a top conductor, and a barrier layer adjacent the bottom conductor; and
wherein the bottom conductor comprises a non-uniform upper surface.
2. The memory device of
3. The memory device of
4. The memory device of
5. The memory device of
6. The memory device of
7. The memory device of
8. The memory device of
9. A tunnel junction for use in a memory element, comprising:
a bottom conductor comprising an upper surface;
a top conductor;
a barrier layer disposed between the bottom conductor and the top conductor; and
wherein the barrier layer comprises a non-uniform surface.
10. The tunnel junction of
11. The tunnel junction of
12. The tunnel junction of
13. A method of producing a tunnel junction for use in a memory element, comprising:
providing a bottom conductor;
creating a non-uniform surface on the barrier layer;
depositing a barrier layer on the non-uniform upper surface; and
depositing a top conductor such that the barrier layer is disposed between the bottom conductor and the top conductor.
14. The method of
15. A memory element comprising:
an anti-fuse comprising a bottom conductor, a top conductor, and a barrier layer of non-uniform thickness therebetween;
an isolator element in series with the first tunnel junction; and
wherein the barrier layer has a dielectric breakdown voltage of between 2 and 3 volts.
16. The memory element of
The invention relates generally to memory devices, and, more particularly, to resistive memory elements and arrays for memory devices.
A resistive memory element is typically characterized by the capability of assuming one of two distinct resistance states at any one time. Data is stored in the element based on the resistance state of the element. Typically, a logic “1” is characterized by a high resistance, while a logic “0” is characterized by a low resistance.
A typical resistive memory element is an anti-fuse memory element. An anti-fuse memory element, as the name implies, functions in an opposite manner to a fuse. An anti-fuse element normally has a very high resistance, typically an open circuit, unless and until a program voltage is applied to the element. When a sufficient voltage is applied to an anti-fuse memory element, the element breaks down and the resistance of the element is reduced to a very low resistance, typically a short circuit. Like a blown fuse, once an anti-fuse memory element is shorted, it is typically impossible or impractical to cause it to open again. Therefore, anti-fuse memory elements are typically referred to as a write-once memory elements, or one-time programmable (OTP) memory elements.
Resistive memory elements are typically arranged in a memory array formed by a plurality of conductive traces arranged in rows and columns. The conductive traces extending along the rows of the array are generally referred to as “word lines” and the conductive traces extending along the columns of the array are generally referred to as “bit lines.” The word lines and bit lines are typically oriented in an orthogonal relationship to each other. A resistive memory element is formed at each intersection (i.e., cross-point) of a word line and a bit line.
Resistive memory devices are typically formed using integrated circuit processing techniques employing various combinations of material depositions, shape definitions using photolithography, and material removal (etches), as known to persons skilled in the art. As noted above, arrays of resistive memory devices are typically formed by arranging a plurality of generally parallel word lines in a generally orthogonal relationship with a plurality of generally parallel bit lines. Each of the word lines is of a generally uniform width, as is each bit line.
The word lines are typically formed by depositing a layer of a metal conductor material, followed by a photolithography step to define the width of the lines and the distances between the conductors, followed by an etch step to remove the conductor material from the spaces between the lines. The bit lines are typically formed in the same fashion, and then are disposed orthogonally to the word lines. Since a resistive memory device, such as an anti-fuse, is formed at each intersection of a word line and a bit line, it is desirable to configure the widths of the word lines and bit lines as narrowly as possible to increase the density of resistive memory devices in an array.
The amount of voltage required to “write to,” or program, a memory element depends on the thickness of the barrier layer. Therefore, in order to lower the required current or voltage, the thickness of the barrier layer must normally be reduced.
Therefore, it can be seen that it is desirable to have methods and devices for reducing the amount of voltage to program a memory element without increasing the potential of encountering shorts within the element.
The present disclosure relates to resistive memory elements. In one embodiment, a resistive memory element comprises a first tunnel junction having a bottom conductor, a top conductor, a barrier layer adjacent the bottom conductor, an isolator element in series with the first tunnel junction, and wherein the bottom conductor includes a non-uniform upper surface.
The present disclosure further relates to a method producing a tunnel junction for use in a memory element. In one embodiment, the method comprises providing a bottom conductor, creating a non-uniform upper surface on the bottom conductor, depositing a barrier layer on the non-uniform upper surface, and depositing a top conductor.
The present invention, as defined in the claims, can be better understood with reference to the following drawings. The components within the drawings are not necessarily to scale relative to each other, emphasis instead being placed upon clearly illustrating the principles of the present invention.
Referring now to the figures, wherein like reference numerals indicate corresponding components throughout the several views, an example of a resistive memory device 100 including a memory element 102 with a non-uniform barrier layer 124 (
As is further illustrated in
The row control circuit 110 includes a plurality of switches for selectively applying a programming, or write voltage Vw, to rows containing a selected memory cell 102 during write processes, or for applying a read potential Vr during read processes. Similarly, the column control circuit 108 can include a plurality of switches for coupling selected bit lines 104 containing selected memory elements 102 to ground during write processes, or for coupling selected bit lines 104 to the sense amplifier 112 during read processes.
Referring also to
Referring also to
The barrier layer 124 can be formed from insulator materials, a multi-layer stack of insulator materials separated by conducting materials, a matrix of insulating material containing dispersed conductive inclusions, amorphous and crystalline semiconductor materials, phase change materials, combinations of a multi-layer stack of Si and silicide-forming metals, etc. Insulator materials include SiOX, SiNX, SiOXNY, AlOX, TaOX, TiOX, AlNX and the like; amorphous and crystalline semiconductor materials include silicon (Si), germanium (Ge), alloys of Si and Ge, InTe, SbTe, GaAs, InSe, InSb, and the like; phase change materials include alloys containing at least two elements selected from Si, Ge, arsenic (As), selenium (Se), indium (In), tin (Sn), antimony (Sb), tellurium (Te), lead (Pb), bismuth (Bi), and the like; silicide-forming metals include tungsten (W), platinum (Pt), palladium (Pd), cobalt (Co), nickel (Ni), titanium (Ti), and the like and alloys thereof. The thickness of the barrier layer 124 may be set to an arbitrary range depending on the materials used as well as the circumstances. For instance, if appreciable current flow is desired through the anti-fuse in a pre-breakdown condition, then the insulator thickness may be chosen so that significant quantum mechanical tunneling current can flow at a modest voltage.
As noted above, the anti-fuse is an element that has an initial high resistance and changes to a relatively low resistance when a critical voltage is applied. The mechanism that achieves the different resistive states is different for different materials. For example, anti-fuses formed from phase change materials have a high resistance when in an amorphous state and a low resistance when in a crystalline state. Also, anti-fuses formed from multi-layer Si and silicide-forming metals have a high resistance when the multi-layer has not been converted to silicide and a low resistance when the multi-layer has been converted to the silicide. In both cases, many orders of magnitude separate the high and low resistance states.
As another example, if an insulator type of anti-fuse is used, up to the critical voltage VC, current passes through the insulating barrier layer 124 of the metal-insulator-metal structure by electron tunneling, and the specific resistance of the element can be rather large, for example, on the order of 107 Ω-μm2. However, beyond the critical voltage VC, the barrier breaks down due to metal migration through the insulator, and the specific resistance of the element can drop to below 100 Ω-μm2. Similar current transport and breakdown mechanisms are operative in layered insulators and insulators containing conductive inclusions.
Preferably, once the non-uniform upper surface 123 is prepared, the barrier layer 124 is created using one of the aforementioned materials. In various embodiments of the present invention, an exemplary average thickness of the barrier layer 124 may be approximately 10-30 angstroms. As well, embodiments are envisioned wherein the opposing sides of the barrier layer 124 are separated by as little as 5 angstroms or less. These dimensions permit critical voltages, Vc, in the range of between 2 and 3 volts, for example, to cause the barrier layer 124 to break down, thereby changing the resistive state of the anti-fuse. Existing anti-fuse. elements require critical voltages of between 5 and 30 volts. The reduced range of critical voltages for embodiments of the present invention allows for reduced energy requirements when programming the memory elements.
As well, various embodiments of the present invention reduce the potential for undesired shorts occurring in the barrier layer 124 during manufacture. In an existing anti-fuse element, in order to lower the required critical voltage, the thickness of the barrier layer normally must be reduced, and this reduction can lead to undesired shorts in the barrier layer. In contrast, to utilize similar critical voltages for various embodiments of the present invention, high electric field breakdown regions formed at the tips of the non-uniform upper surface 123 of the bottom conductor allow the minimum thickness of the barrier layer 124 of the present invention to actually be thicker than the thickness of the uniform barrier layer of the existing anti-fuse, yet have approximately the same critical voltage. These high electric field breakdown regions contrast existing anti-fuse structures having planar configurations, wherein high electric field breakdown regions are not isolated to specific locations on the structure.
The non-uniform upper surface can be created using a variety of methods. For example, a strain-lattice mismatch can be used. If one material is grown on top of another, and the two materials have different lattice constants, strain will exist at the interface of the two materials. This interfacial strain can cause the second layer to break into grains to relieve the strain. The higher the strain, the smaller the grains. These grains tend to grow in columnar growth structure, which tends to have “domed” peaks. The domed grains create a rough surface. The larger the lattice mismatch, the smaller the grains, the steeper the dome, and the greater the surface roughness of the material.
Another method includes the material property known as “wetting.” Basically, a material that “wets” well gives a uniform coverage when placed on top of a first material. A material that “wets” well will have a low surface roughness. However, a material that does not “wet” well will tend to form islands when placed on a first material. These islands grow until they coalesce, and then form grains as the material is grown thicker. A material that exhibits island growth will have a high surface roughness. The more grains a material forms the greater the surface roughness. Processing techniques may be employed to increase the number of nucleation sites for island growth. Lattice mismatch, temperature, and growth rate all contribute to the character of island growth.
Ion bombardment can also be used to roughen a surface. The effectiveness of ion bombardment can be enhanced if the material to be roughened has a texture with coherent lattice planes. The ion bombardment can be directed at such an angle so as to selectively etch some planes more than others, increasing the surface roughness. As well, chemical etchants can be used to enhance surface roughness. Some chemical etchants preferably etch a grain boundaries. Chemical etchants may be used either alone or in combination with one of the previously noted methods.
After the non-uniform surface is prepared, a barrier layer is then deposited on the bottom conductor, as shown in block 606. In a preferred embodiment, the barrier layer consists of one of the anti-fuse materials discussed hereinabove. A top conductor is deposited on the barrier layer, as shown in block 608. As with the bottom conductor, various methods may be used to form the top conductor. As previously discussed with regard to various preferred embodiments, the bottom conductor is not the only layer on which the non-uniform surface can be created (i.e., the silicon layer in
It will be apparent to those skilled in the art that many modifications and variations may be made to the preferred embodiments of the present invention, as set forth above, without departing substantially from the principles of the present invention. All such modifications and variations are intended to be included herein within the scope of the present invention, as defined in the claims that follow.